1. Field of the Invention
The present invention relates to sheet-feeding mechanisms of printers, and particularly, to a sheet-feeding mechanism of a printer that conveys a recording sheet with high accuracy to print a high-quality image.
2. Description of the Related Art
A conventional sheet-feeding mechanism of a printer will be described with reference to FIGS. 7 and 8. Japanese Unexamined Patent Application Publication No. 2002-144616 discloses an example of a printer 31 equipped with a conventional sheet-feeding mechanism. The printer 31 is, for example, a thermal-transfer printer and includes a platen roller 32 and a thermal head 34 that is press-contactable with the platen roller 32 via a recording sheet 33. The recording sheet 33 and the thermal head 34 have an ink ribbon (not shown) extending therebetween.
A first sheet-feeding roller 35 and a first pressing roller 36 defining a sheet-feeding unit are disposed upstream of the thermal head 34 with respect to a direction in which the recording sheet 33 being printed is conveyed, as indicated by an arrow A. The first sheet-feeding roller 35 and the first pressing roller 36 are capable of coming into and out of contact with each other. On the other hand, a second sheet-feeding roller 37 and a second pressing roller 38 are disposed downstream of the thermal head 34 such that the second sheet-feeding roller 37 and the second pressing roller 38 are capable of coming into and out of contact with each other.
When the recording sheet 33 is nipped between the first pressing roller 36 and the first sheet-feeding roller 35 rotating counterclockwise, the recording sheet 33 is conveyed in the direction of the arrow A and thus becomes nipped between the thermal head 34 in a head-down state and the platen roller 32.
In this state, a plurality of heater elements (not shown) included in the thermal head 34 is selectively heated to heat-transfer the ink in the ink ribbon to the recording sheet 33. As a result, a predetermined image is printed on the recording sheet 33.
A front end 33a of the recording sheet 33 conveyed in the direction of the arrow A in the printing process pushes upward the second pressing roller 38 in press-contact with the second sheet-feeding roller 37 as shown with a double-dashed line. Thus, the recording sheet 33 becomes nipped between the second pressing roller 38 and the second sheet-feeding roller 37 rotating counterclockwise, whereby the predetermined image is printed on the top surface of the recording sheet 33.
However, during the printing process in a conventional sheet-feeding mechanism of a printer, the recording sheet 33 being printed is stopped momentarily while the front end 33a of the recording sheet 33 formed of heavy paper hits against the second sheet-feeding roller 37 and the second pressing roller 38 in order to push the second pressing roller 38 upward.
This may possibly produce an undesirable line in a section of the printed image corresponding to where the recording sheet 33 is momentarily stopped since the ink in the ink ribbon is transferred to that section in a partially overlapping manner.
On the other hand, referring to FIG. 8, while the recording sheet 33 nipped between the second sheet-feeding roller 37 and the second pressing roller 38 is being conveyed in the direction of the arrow A during the printing process, the nipping force of the first sheet-feeding roller 35 and the first pressing roller 36 is released from a rear end 33b of the recording sheet 33. In this case, the rear end 33b momentarily slides on the outer periphery surfaces of the first sheet-feeding roller 35 and the first pressing roller 36, such that the rear end 33b of the recording sheet 33 being conveyed in the direction of the arrow A is momentarily pushed in the direction of the arrow A by a small amount.
This temporarily increases the conveying rate of the recording sheet 33 being printed, which may produce a white line in a section of the printed image where the ink in the ink ribbon is not transferred.